1. Field of the Invention
This invention relates to a process for preparing urethanes by reacting a solution of a nitrogen-containing organic compound and a hydroxyl-containing organic compound with carbon monoxide in the presence of a rhodium catalyst.
2. Description of the Art
Various patents have disclosed methods for carbonylating nitrogen-containing organic compounds--e.g., nitro compounds, amines, azo- and azoxy compounds,--to urethanes in the presence of a platinum group metal-containing catalyst usually a palladium or rhodium-containing catalyst and most often a palladium or rhodium halide-containing catalyst. Generally, a co-catalyst (promoter) has been needed in combination with the platinum group metal-containing catalyst in order to obtain improved rates of reaction. The vast majority of prior art processes use, as a co-catalyst, a halide salt of a metal which is redox-active under the reaction conditions, usually iron, and most often iron chlorides. The co-catalyst is used in substantial molar excess compared to the main catalyst in order to obtain the desired reaction rate. These large quantities of redox-active metal halides are troublesome to separate from the reaction product and cause substantial corrosion problems.
A few references have taught the addition of a primary amino compound (and/or related compounds, such as urea, biurets, and allophanates) to further improve the rate and selectivity of reactions catalyzed by a platinum group metal compound in combination with a redox-active metal halide-cocatalyst. U.S. Pat. No. 4,178,455 discloses that, in a process for converting nitroaromatic to urethane catalyzed by a platinum, palladium, rhodium, or ruthenium compound and a Lewis-acid promoter, the rate and selectivity are improved by adding to the reaction, an organic primary amino compound, a urea compound, a biuret compound, an allophanate compound, or a mixture thereof. The preferred Lewis acid promoters are redox-active metal salts, especially iron chlorides. This patent illustrates (by example) only palladium catalysts with iron chloride promoters. A careful study of the examples reveals that the starting nitroaromatic and the primary amino compound (or related compound) are both converted, in net, to urethane. Thus, when the primary amino compound or urea compound contains the same aryl group as the starting nitroaromatic compound the reported yield of urethane, based on only the nitroaromatic converted, exceeds 100%. This patent also teaches the use of tertiary amines, e.g. pyridine, in large molar excess compared to the palladium catalyst to prevent corrosion. See also U.S. Pat. No. 4,186,269 wherein a tertiary amine, e.g. pyridine, in large molar excess is utilized to suppress corrosion in a process utilizing a catalyst system comprising (1) palladium, ruthenium, rhodium or compounds thereof, and (2) a Lewis Acid, e.g. ferric chloride. Similarly, U.S. Pat. Nos. 4,219,661; 4,262,130; and 4,339,592 teach palladium catalysts with iron oxide and iron chloride co-catalysts in which addition of tertiary amines is one embodiment. Pyridine is the demonstrated tertiary amine used in large molar excess relative to palladium and the stated purpose of the pyridine is to decrease undesirable secondary reactions of the alcohol reactant. No effect of the tertiary amines on the activity of the catalyst towards urethane synthesis is disclosed.
U.S. Pat. No. 4,297,501 discloses a process in which mixtures of a primary amine and a nitroaromatic are carbonylated to urethane with a Group VIII noble metal compound and an oxychloride compound capable of undergoing redox reactions. Alternatively, an oxide compound capable of undergoing redox reactions in combination with an anionic chloride compound is used as the promoter system. Illustrated, by example, are PdCl.sub.2 and RhCl.sub.3, as Group VIII noble metal compounds, and the oxides and chlorides of vanadium and iron. In the preferred embodiment of U.S. Pat. No. 4,297,501, the nitroaromatic corresponds to the primary amine, and the patent teaches the following reaction stoichiometry: EQU 2RNH.sub.2 +RNO.sub.2 +3CO+3R'OH.fwdarw.3RNHCO.sub.2 R'+2H.sub.2 O(1)
U.S. Pat. No. 4,297,501 further teaches that when nitroaromatic is present in excess of the 1:2 ratio relative to amine, the remaining nitroaromatic is converted to urethane by the following reaction stoichiometry: EQU RNO.sub.2 +3CO+R'OH.fwdarw.RNHCO.sub.2 R'+2CO.sub.2 ( 2)
It can be seen from the above equations that when primary amine is initially present, in processes which convert nitroaromatic to urethane using Group VIII noble metals in combination with redox-active metal halide co-catalysts, the primary amine is, in net, consumed to also make urethane. (See equation (1) above). Once the primary amine is consumed to low levels, any remaining nitrobenzene is converted to urethane via reaction equation (2) above. Since the primary amine is already consumed to low levels, it is no longer available to favorably influence the rate of the process according to said reaction (2).
This patent does not teach the addition of a tertiary amine to the disclosed catalyst and promoter.
U.S. Pat. No. 4,304,922 similarly discloses a process in which mixtures of N,N'-diaryl urea and nitroaromatic are carbonylated to urethane with the same catalyst/co-catalyst systems of U.S. Pat. No. 4,297,501. Illustrated by examples are PdCl.sub.2, RhCl.sub.3, IrCl.sub.3, PtCl.sub.4 and RuCl.sub.3 as Group VIII noble metal compounds. Iron oxychloride and several other redox active metal oxides and chlorides are illustrated as co-catalysts. In examples in which redox active metal oxides are used, anilinium hydrochloride is also added to provide active anionic chloride. In the preferred embodiment of this patent, the N,N'-diaryl urea and nitroaromatic have the same aryl groups, and the patent teaches that the following reaction stoichiometry is obtained: EQU 2RNHCONHR+RNO.sub.2 +3CO+5R'OH.fwdarw.5RNHCO.sub.2 R'+2H.sub.2 O(3)
It is known that N,N' diarylureas react with alcohols to produce urethane plus amine; see for Example U.S. Pat. No. 2,409,712, wherein the following reaction is disclosed: EQU RNHCONHR+R'OH.fwdarw.RNHCO.sub.2 R'+RNH.sub.2 ( 4)
It can be seen that once this occurs under the reaction conditions, the same process as U.S. Pat. No. 4,297,501 is obtained according to equation (1) above. (Twice equation (4) plus equation (1) equals equation (3)). It can further be seen that both N,N'-diaryl urea and arylamine are, in net, consumed in the process to make urethane. Example 11 of U.S. Pat. No. 4,304,922 illustrates that when RhCl.sub.3 is used as catalyst in combination with iron oxychloride as co-catalyst, nitrobenzene and N,N'-diphenylurea (1:2 molar ratio) are both consumed (100% and 99% conversion, respectively) to give urethane product 99% selectivity based on nitrobenzene plus N,N'-diphenylurea).
Again, like U.S. Pat. No. 4,297,501 there is no teaching of the advantage of adding a tertiary amine to the disclosed process.
Japan Kokai No. 55-7227 discloses a process in which molecular hydrogen is added, to a process for carbonylating nitroaromatic, in the presence of a palladium catalyst, to increase the reaction rate. The description of the invention specifies a palladium catalyst, accompanied by promoters such as tertiary amines, iron and vanadium compounds, and chlorine ions. All illustrated examples use a supported palladium-selenium on carbon catalyst promoted with pyridine and either FeCl.sub.2 or VOCl.sub.3 (these are redox-active metal chlorides). The patent teaches that the addition of hydrogen causes hydrogenation of a fraction of the nitroaromatic to generate the corresponding primary arylamine in situ. The process is thus generically similar to that of U.S. Pat. No. 4,178,455, discussed above, which illustrates by example the addition of primary arylamine to a reaction with a supported palladium catalyst promoted with FeCl.sub.3. Thus, it may be concluded that primary amine generated from hydrogen will in net be consumed in the reaction to make urethane. Indeed, Japan Kokai No. 55-7227 teaches that any primary amine remaining at the end of a reaction can be returned to another reaction with more nitroaromatic, in which case the primary amine is easily converted to urethane.
In U.S. Pat. No. 4,474,978 a process is disclosed for converting a nitroaromatic to a urethane in the presence of a primary amine and a catalyst system based on palladium complexed with Group VA-chelate ligands, including bis-tertiary amino-containing ligands. The patent teaches that the redox active metal chloride and related co-catalysts are no longer needed when the above ligands are used. But, this patent does not suggest the use of rhodium in combination with said ligands and also teaches that the primary amine and/or urea are co-converted with the nitroaromatic to urethane. Thus, the process, in net, consumes added amine or urea. See the examples disclosed in the patent.
Thus, it is clear that, in the processes cited above, as the primary amine and/or urea compound is converted, in net, to urethane, its concentration decreases and its effects on reaction rate and selectivity must also decrease. Eventually, as nitroaromatic continues to be converted, either in a batch process or in a continuous process (with recycle of the remaining amine), the primary amine will be consumed to a low concentration. In order to maintain the improved rates and selectivities, which are obtained by the original addition of primary amine, urea, hydrogen, etc., it is necessary to provide additional primary amine, urea, hydrogen, etc. as the primary amine is consumed.
A few references teach the use of rhodium catalysts, in the absence of a redox-active metal co-catalysts, for the carbonylation of nitrogen-containing organic compounds to urethanes. However, these references do not teach the initial addition of primary amines, ureas, hydrogen, etc. to obtain improved activity. For example, U.S. Pat. No. 3,338,956 discloses a metal carbonyl catalyst of Group VIA, VIIA, or VIIIA for this reaction. The only such catalyst exemplified, however, is rhodium chlorocarbonyl and the rates of reaction are relatively slow. Moreover, this reference does not teach the advantages of adding a tertiary amine to the disclosed carbonylation process.
U.S. Pat. No. 3,993,685 teaches the addition of tertiary amines, especially pyridine, to platinum group metal catalysts to obtain improved activity in the absence of redox-active metal co-catalysts. Rhodium chloride and hydridocarbonyltris(triphenylphosphine) rhodium in combination with pyridine are exemplified. In this reference it is clearly pointed out that a high concentration or excess of the tertiary amine is desirable, the tertiary amine serving as both catalyst and solvent. (Note that the suggested tertiary amines are not bis(tertiary amines)).
U.S. Pat. No. 4,052,437 discloses the use of rhodium oxide as catalyst, preferentially in nitrilic solvent. Rh.sub.6 (CO).sub.16 as a catalyst is also exemplified in this patent. There is no suggestion that the initial addition of a primary aryl amine to the process disclosed in this patent would improve the rate. Also, there is no suggestion of the advantage of adding a tertiary amine to the catalyst disclosed therein.
An article in the Journal of Organic Chemistry 37, 2791 (1972) describes a reaction in which nitro-benzene in the presence of ethanol is carbonylated in low yield (&lt;10%) to urethane with a catalyst comprising Rh.sub.6 (CO).sub.16 in pyridine solvent. The major product was aniline. Again, like U.S. Pat. No. 3,993,685, above, the pyridine is used in high concentration or excess to enable its function as a solvent for the reaction.
None of the above cited art, which discloses the use of rhodium catalysts (in the absence of redox-active metal co-catalysts) for the carbonylation of nitro-organics to urethanes, discloses the initial addition of primary amine, urea, hydrogen, etc. Moreover, the effect of initially adding primary amine to such catalysts is not predictable. Finally, the result obtained by adding a primary amine to a rhodium catalyst system essentially free from redox-active metal components, is substantially different from the result obtained when a primary amine is added to either Group VIII metal catalysts (including rhodium and palladium) in the presence of redox active metal co-catalysts or certain palladium catalysts in the absence of redox active metal co-catalysts.
There are references, which disclose the conversion of nitroaromatic to urethane in the presence of a platinum metal catalysts and in which the primary amine is not in net consumed. See copending patent applications Ser. Nos. 532,784; 532,785 and 707,885, filed Sept. 16, 1983, Sept. 16, 1983 and Mar. 4, 1985, respectively. All three applications were filed in the names of Grate, Hamm, and Valentine and all are entitled "Process for the Preparation of Urethanes". Also, see patent application Ser. No. 744,951, filed June 17, 1985, in the names of Grate and Hamm, and entitled "Process for the Preparation of Urethanes."
It is an object of this invention to provide a process for the conversion of nitro-aromatic to urethane in good rate and selectivity, without requiring continual addition of primary amine, urea, hydrogen, etc. to maintain the rate and selectivity.
It is a further object of this invention to effectively carry out the above process in the absence of redox-active metal halide co-catalysts.
It is also an object of this invention to effectively carry out the above process in the absence of a large molar excess of tertiary amine as compared to the catalyst.